This invention relates to calibration of measurement systems of the type in which a sensor, the electrical resistance of which varies as a function of the parameter being measured, is electrically connected in one arm of a bridge circuit. The invention is particularly applicable to oceanography systems in which thermistors are used to measure water temperature, such as the system disclosed in U.S. patent application Ser. No. 608,030, filed Aug. 27, 1975 by Ralph G. Washburn, the disclosure of said application being hereby incorporated herein by reference.
In such oceanography systems the sensor is often connected in the bridge circuit arm through a transmission line sufficiently long to require consideration of its electrical impedance in the bridge design, and a second long transmission wire is connected in a second arm of the bridge, the two wires having equal resistances R.sub.c and being arranged so that any resistance-changing conditions affect both wires equally during operation of the system. A reference voltage is introduced into the bridge, and active control circuitry is provided to balance the bridge by controlling the currents in the bridge arms. The bridge current is then measured to provide a readout indicative of the value of the parameter being measured.
It is often desirable that the output of such measurement systems be provided in frequency form (e.g., a sine wave, or a pulse train) capable of modulating a radio frequency transmitter or producing digital data for computer interfacing. Output circuitry is provided to convert the bridge output into a system output in accordance with any desired output function. For example, the output circuitry may include a voltage controlled oscillator for providing a frequency output, as well as circuitry (not described herein) for ultimately converting the output back to a non-frequency form.
In one important class of systems to which the invention is applicable, the bridge output, although in general dependent upon the sensor resistance and the reference voltage, is independent of the reference voltage at one end of a range of values of the parameter being measured. One example of such a system is that disclosed in said application Ser. No. 608,030, wherein a grounded power source is used to supply reference, scale offset, and operating voltages to the circuit.